This invention relates to heavy duty pneumatic radial tires, and more particularly to an improvement of tread groove shape in the tire of this type suitable for use in heavy vehicles such as truck, bus and the like running not only on good roads at high speed but also on bad road inclusive of general unpaved road and construction sites.
In general, radial tires using metal cords as a belt reinforcement have several merits such as wear resistance, puncture resistance and the like as compared with the usual bias tires. This is because a stiff belt is disposed between the tread rubber and the carcass ply, but are poor inride feeling due to the rigid reinforcing effect with the belt. Therefore, the development of such radial tires are particularly advanced for use on good roads aside from bad roads. Lately, the demand for these tires is conspicuous with the remarkable improvement of road situations such as the development of expressway networks and the like.
Considering the traction and braking performances as well as general wear resistance and resistance to heat buildup for such applications, the tread of the tire is provided with circumferential ribs defined by main zigzag grooves extending in the circumferential direction of tire, which is generally called as a rib-type tread pattern. Although such ribs are usually continuous in the circumferential direction of tire, there are some cases that the rib is further provided therein with a fine groove extending in the circumferential direction and that the rib is discontinuous in the circumferential direction by separating with transverse grooves arranged along the widthwise direction of tire. In any case, peculiar abnormal wears occur in the rib when continuously running over a long distance at a high running speed.
One of these abnormal wears is mainly a so-called railway wear, which starts from a side edge portion of a circumferential rib 3 defined by tread grooves 1, 2 of a tire T and gradually spreads in the circumferential direction over a region with a stepwise difference .delta. and a width w as shown in FIG. 1 when the tire goes on road continuously straight. Another of the abnormal wears is mainly a so-called waved wear, which starts from an outer side edge portion of an outermost circumferential rib 4 viewing from the axial direction of tire T and discontinuously produces a stepwise .delta.' along the circumferential direction of tire to locally wear the rib 4 as shown in FIG. 2 when the tire is affected by cornering.
These abnormal wear patterns not only render the appearance of the tire T awkward, but also adversely affect tire performance, particularly traction performance, braking performance and cornering performance, which largely reduce the tire life.
Moreover, the vehicles provided with tires for use on a good road surface are obliged to run on a bad road surface locally included in such a good road. For instance, it is indispensable to drive trucks or the like into construction sites, so that there are many chances of picking up foreign matter such as stones scattered on road surface or the like by tread grooves.
In the heavy duty pneumatic radial tire, the movement of tread is suppressed at its ground contact area by the stiff tread reinforcement as mentioned above, so that such tires are apt to cause stone pick-up and if caused, the picked stones are strongly retained within the grooves and are difficult to be discharged therefrom. As a result, the retained stone repeatedly strikes on the base rubber above the tread reinforcement or the belt at the bottom of the circumferential groove every the tire rotation, which produces the breaking of the base rubber and hence the internal damage of the reinforcement structure. Particularly, when the belt is composed of metal cords, rust corrosion is caused by the penetration of water from the damaged portion of the base rubber to separate the cords from rubber, and finally fatal damage may be produced in the durable use life of tire inclusive of tire retreading.
Heretofore, there have been various attempts for individually solving the aforementioned two drawbacks, i.e. the occurrence of abnormal wears and the problem of stone pick-up, but these drawbacks cannot simultaneously be solved by such attempts because all cross-sectional shapes of the circumferential grooves are set substantially same in these attempts.
Alternatively, as disclosed in U.S. Pat. No. 3,763,911, the cross-sectional shape of the circumferential groove adjacent to the center rib is made so that the width of the center rib gradually reduces inward in the radial direction of tire and a protrusion is disposed on the groove bottom independently of the rib, whereby it is attempted to reduce the railway wear and at the same time prevent the stone pick-up in these circumferential grooves. In this case, however, the shape of the groove becomes complicated and is difficult in its formation, and also the above drawbacks are not effectively solved as an overall surface of the tread.
It is, therefore, an object of the invention to provide tires which can simultaneously solve the above two problems of two kinds of abnormal wear and stone pick-up, which are unavoidable in the tire used on good road surfaces including bad road surfaces, over a whole contact area of tread advantageously, simply and effectively, i.e. tires capable of realizing uniform wear over the whole contact area of the tread and reduction of total stone pick-up number.
The inventors have found the following in connection with a tread pattern as shown in FIG. 3:
(1) When an inclination angle of a groove wall in all of tread main grooves 1, 2 with respect to a normal line drawn from the outer surface of the tread and passing each edge of the grooves is set relatively large, railway wear is particularly apt to occur in an outer edge portion 3.sub.-2 of a circumferential rib 3 during the running on good road; and PA1 (2) When the inclination angle of the groove wall in all main grooves 1, 2 is set relatively small, waved wear is apt to occur in an outermost rib 4 during the running on good road, while the stone pick-up is apt to occur in the tread main grooves 1, 2 during the running on bad road and also the degree of stone pick-up is large in the tread main groove 1 located at the center portion of the tread.
On the basis of the above facts, the inventors have aimed at such a point that abnormal wears such as railway wear, waved wear and the like and stone pick-up are apt to occur in different positions of the tread. It has been confirmed that the aforementioned problems are simultaneously solved by taking the shape of the groove suitable for preventing the occurrence of individual problem in each position, whereby it is made possible to provide tires advantageously adaptable for use on both good and bad roads.
According to the invention, there is the provision of in a heavy duty pneumatic radial tire comprising a carcass of a substantially radial construction composed of at least one rubberized ply layer containing cords embedded therein and a belt superimposed about said carcass for stiff reinforcement beneath a tread and composed of at least two rubberized ply layers each containing metal cords embedded therein, said metal cords of which being crossed with each other at a relatively small angle with respect to the circumferential direction of tire, and said tread being provided with a plurality of continuous or discontinuous zigzag circumferential ribs defined along the widthwise direction of tire by at least three, substantially zigzag main grooves extending circumferentially of said tread, said main grooves comprising one or a pair of central circumferential grooves located at a substantially central region of said tread and a pair of outside circumferential grooves defining each of outermost ribs of said tread, the improvement wherein said central circumferential groove has such a symmetrical cross-sectional shape with respect to a center line of said groove that an inclination angle of a groove wall of said groove with respect to a normal line drawn from an outer surface of said tread and passing an edge of said groove in the cross section perpendicular to said groove wall is made relatively large in a region extending from the groove bottom to at least 50% of groove depth, and said outside circumferential groove has such an unsymmetrical cross-sectional shape with respect to a center line of said groove that an inclination angle of an outer groove wall of said groove in the rotation axial direction of tire is made relatively large and an inclination angle of an inner groove wall of said groove in a region extending from the outer surface of said tread to at least 10% of groove depth is made smaller than that of said outer groove wall.
According to one embodiment of the invention, the tread is divided into a plurality of continuous or discontinuous zigzag circumferential ribs along the widthwise direction of tire by at least five, substantially zigzag main grooves extending circumferentially of the tread, and these main grooves comprises one or a pair of central circumferential grooves located at a substantially central region of the tread, a pair of outside circumferential grooves defining each of outermost ribs of the tread and at least a pair of middle circumferential grooves each arranged between the central circumferential groove and the outside circumferential groove. In this case, the middle circumferential groove preferably has such a symmetrical cross-sectional shape with respect to a center line of the groove that the groove wall is a substantially polygonal line wherein an inclination angle of a radially inner portion of the groove wall is larger than that of a radially outer portion of the groove wall.